1. Field of the Invention
The present invention concerns a method for supercharging internal combustion engines and an internal combustion engine operating by this method.
2. Description of the Prior Art
The engines of passenger cars are operated on the average with samll mean effective pressure p.sub.me and low average speeds n.sub.m compared with the corresponding values at the nominal power. Typical figures for a diesel engine, for example, are mean effective pressures of 2 to 3 bar and average speeds of 2,000 r.p.m. for a maximum speed of approximately 4,000 r.p.m.
For mixed operation due to town and country driving, passenger car engines operate 80% of the time with a p.sub.me below 5 bar and two-thirds of the fuel consumption also occurs within this range. The high values of p.sub.me are only required for brief acceleration maneuvers but they must then be rapidly available at the required level. It is only at high motorway speeds and on steep mountain stretches that high mean effective pressures and speeds are demanded from the engine for longer periods.
For these reasons, supercharged passenger car engines must meet different operational behavior requirements than those of supercharged truck engines, for which high efficiency at high load is demanded, whereas from what has previously been stated the fuel consumption of car engines should be as small as possible at low loads, a higher consumption at large loads being accepted in order to meet this requirement.
The rapid increase in torque at low p.sub.me required for fast acceleration maneuvers is not easy to attain with exhaust gas turbochargers. This is because the exhaust gas temperature of diesel engines is low in this range, lying generally between 150.degree. and 250.degree. C., and consequently the energy available to the turbine is also small. Under these circumstances, the turbocharger operates with an overall efficiency of a maximum of 30%. The boost pressure is then lower than the exhaust gas pressure in front of the turbine and this implies negative scavenging work, i.e. a loss of efficiency. The negative pressure difference is then usually between 0.05 and 0.1 bar. It follows that the fuel consumption of a turbocharged engine in this range is also higher than that of a naturally aspirated engine, for which the suction pressure and the exhaust back pressure are approximately equal.
The losses due to the negative scavenging work under partial load increase with increasing pressure ratio across the turbine. In a certain Otto engine, the exhaust back pressure is reduced by blowing down exhaust gas before the turbine in order to reduce these losses. This measure is normally only applied in the upper speed range because the turbocharger is, as is known, matched with respect to boost pressure to the air requirement of the engine in the range mentioned of p.sub.me mainly used but characteristically provide pressures which are too high in the upper speed range and which are reduced by blowing down to a value which the engine can tolerate without being overloaded. Using this measure, an approximation is attained to the actual boost pressure variation which would be theoretically desirable for the engine. This variation, however cannot be considered ideal because there is a lack of torque at low engine speeds--the so-called turbo lag. Particularly disadvantageous with respect to driving operations is the fact that the turbocharger must first be brought up to speed during an acceleration maneuver before the necessary steady engine torque becomes available. This phenomenom is known as the "turbo lag".
A disadvantageous feature of this blowing-down under partial load before the turbine entry using an exhaust gas bypass, a so-called "waste gate", is that very little ram pressure builds up in front of the turbine so that the turbocharger speed remains even lower. It thus follows that there is a further delay when accelerating in addition to the already mentioned delayed response of the turbocharger, i.e. the turbo lag, because the desirable ram pressure build up for the rapid running up the turbine only appears after the waste gate is closed. This delay is even greater in the case of turbochargers with high pressure recirculation, because the latter can only be switched off at high p.sub.me, e.g. 5 bar.
In order to avoid the disadvantages of conventional exhaust turbochargers with and without a waste gate, turbochargers have been variously proposed with variable swallowing capacity. In these turbocharger units, referred to as varioturbochargers hereinafter and which may also include variable compressors in some cases, the entry area of the turbine and thus its swallowing capacity is reduced using adjustable nozzle guide vanes, double volute intakes, diaphragm nozzle rings, and slideable nozzle rings, inter alia. The increased build up of ram pressure provides sufficient turbine power so that it runs faster even at low engine speed and develops a high torque. Furthermore, more energy is available for faster acceleration of the compressor in the load range mentioned at the beginning, within which the acceleration behavior of the engine is to be improved using the present invention.
The principle of the same features can also be used in internal combustion engines with other exhaust gas driven supercharger units, particularly with those using gas dynamic pressure wave superchargers with equipment for altering the exhaust gas swallowing capacity, such as a waste gate and recirculation duct. This is so because the exhaust gas temperature and therefore the energy available from the exhaust gases is also small in the low p.sub.me range in the situation where a diesel engine is combined with a pressure wave supercharger. In this case also, the engine must compensate for the negative scavenging work because the exhaust gas pressure in it is usually greater than the scavenging pressure by 0.1-0.15 bar in this operating condition. Such a pressure wave supercharger is indeed clearly superior to an exhaust gas turbocharger with respect to the acceleration capability of its air conveying element which is formed by a rotary vane, but even its response behavior upon change of load and, in particular, its fuel consumption can be improved by regulation according to the invention of the swallowing capacity of the pressure wave supercharger.
In the p.sub.me -n.sub.m diagram of a typical passenger car engine shown in FIG. 1, the field of mean effective combustion pressures with which the vehicle is mainly operated, as mentioned at the beginning, is indicated by reference numeral 1. The range required for motorway and hill-climbing work is indicated by reference numeral 3 and the curve of maximum torque used for acceleration is indicated by reference numeral 2.
FIG. 2 shows the variation of the mean effective pressure p.sub.me on an engine with a conventional turbocharger with a waste gate and on an engine with a varioturbocharger with a waste gate. In this diagram, the full line curve 4 shows the pressure variation p.sub.me (n.sub.m) of a varioturbocharger and the dashed curve 5 shows the corresponding variation of a normal turbocharger with constant turbine swallowing capacity. Up to the speed corresponding to the point 6, the curve 4 for the varioturbocharger lies above the curve 5 for the normal supercharger and it is only beyond this point that the latter utilizes the exhaust gas stream as well as the varioturbocharger and the two curves become coincident. Since, in the case of the varioturbocharger, the boost pressure permissible for the engine is attained at a lower engine speed, corresponding to point 7, than in the case of the normal turbocharger, the swallowing capacity of the varioturbocharger must be continually increased, in fact along the line 8 from the speed corresponding to point 7, whereas, like in the case of a normal turbocharger, the waste gate only begins to open along the line 9 respectively from the point 6. However, point 6 can also be chosen to be different from that of the normal turbocharger engine.
FIG. 2 thus shows an engine with a varioturbocharger has a higher torque in the sample chosen in the interesting speed range between about 1,000 and 2,000 r.p.m. and thus offers better acceleration.
To complement the statements made at the beginning, the behavior of a conventional turbocharger will be described using the diagram of FIG. 3 and, subsequently, the behavior of an engine with a varioturbocharger, a waste gate and possibly exhaust gas recirculation will be described using FIG. 4.
The two diagrams indicate the relationship between the mass flow m in kg/sec and the pressure ratio .pi..sub.T of the turbine and .pi..sub.v of the compressor for a typical passenger car engine with a waste gate with the assumption that m.sub.m =m.sub.v =m.sub.T. The dashed straight lines S.sub.1,000 to S.sub.4,000 are the swallowing capacity lines of the engine m.sub.v (.pi..sub.v) at the speeds n.sub.m =1,000 to n.sub.m =4,000 r.p.m. The full line parabola-shaped curves T.sub.700, T.sub.550 and T.sub.225 are the turbine swallowing capacity lines m.sub.T (.pi..sub.T) at a gas entry temperature of 700.degree. C., corresponding to a speed of approximately n.sub.m =2,500 r.p.m., at which the waste gate is still just closed, at a gas entry temperature of 550.degree. C., at which the torque M.sub.d is attained at n.sub.m =2,000 r.p.m., and at 225.degree. C. gas entry temperature, i.e. in the part load range with a p.sub.me =2 bar and n.sub.m =2,000 r.p.m. The curves (T+WG).sub.750 and (T+WG).sub.225 show the swallowing capacity lines for the turbine plus an open waste gate at 750.degree. and 225.degree. C. gas inlet temperature. Corresponding to the M.sub.d max curve of the engine, the operating points for the turbine lie on the dashed line between the point 10 of the line for T.sub.700 and the point 11 on the line (T+WG).sub.750 and, in the case of the compressor, on the chain dotted curve between the point 12 and the point 13 on the swallowing capacity line S.sub.4,000. It may be seen that the pressure difference between the compressor outlet and the turbine inlet up to the point 14, the intersection point of the two lines mentioned previously, is positive and then becomes negative up to the maximum engine speed. Thus in the latter range, the pressure upstream of the turbine is higher than that at the compressor outlet.
More important to understanding of the present invention are the relevant relationships of the present invention turbocharger in the partial load range, within which the torque behavior is to be improved using the invention. These are explained using the example of a condition with p.sub.me =2 bar, n.sub.m =2,000 r.p.m. and the temperature T.sub.225 before the turbine. A closed waste gas then produces a .pi..sub.T =1.1 (see point 15). This provides a surplus pressure .DELTA.p before the turbine compared with the boost pressure of 0.07 bar (see line 15-16=.DELTA.p.sub.ow :ow=without waste gate). The fuel consumption increases by approximately 1.6% due to the corresponding scavenging work of the piston. With the waste gate open, the surplus pressure drops to .DELTA.p.sub.mw =0.03 bar and if high pessure recirculation is also used, this surplus pressure with its deleterious effect on efficiency drops still further to .DELTA.p.sub.mw+Rez.
Using the diagram in FIG. 4, the behavior of an engine with a varioturbocharger, a waste gate and with exhaust gas recirculation will now be discussed. Even if the varioturbocharger shows an advance with respect to the torque behavior in the lower speed range compared with a conventional turbocharger, it still has some operational deficiencies which, inter alia, are to be obviated with the present invention. The construction of such an engine is shown in principle in FIG. 5. In this diagram, reference numeral 17 represents a waste gate duct, 18 a recirculation duct available as required and the region 19 in the dashed circle the turbine inlet with variable flow area of a varioturbocharger. There are many proposals for solving varioturbocharger problems, including, inter alia, some with divided exhaust pipes for the application of the impulse design.
In addition to some symbols already familiar from FIG. 3, FIG. 4 includes the signs VT.sub.zu =varioturbocharger closed, i.e. throttled to the minimum flow area and VT.sub.auf =varioturbocharger open, with maximum flow area, which are supplied to the swallowing capacity lines of varioturbocharger VT. The indices 225 etc. in each case refer to the exhaust gas temperature at the turbine inlet.
The maximum torque corresponds to the point 20 and such is already reached at an engine speed of under 2,000 r.p.m. Since the highest permissible boost pressure is thus attained, the opening of the entry section of varioturbocharger now begins and this process is concluded at point 21. The further curve as far as point 22 then behaves like a conventional turbine with constant entry section. One difference is that the flow area when fully open can be arranged to be greater than the area of a conventional turbine for the same engine, so reducing the quantity blown down through the waste gate or, in the extreme case, obviating this altogether. However, a waste gate has been assumed for FIG. 4 and this functions in the range of 22 to 23.
Even at low speeds, a varioturbocharger thus provides the engine with sufficient air for the desired higher torque in the operating range mainly used in a typical passenger car engine, as mentioned in the introduction.
On the other hand, there are driving conditions which are less favorable compared with the normal turbocharger engine. At the partial load point 24 with n.sub.m =2,000 r.p.m and p.sub.me =2 bar, the exhaust gas builds up to .pi..sub.T =1.3 and the excess of exhaust pressure over boost pressure, represented by the line 24-25, attains .DELTA.p.sub.VT =0.15 bar, which causes a 1.85% higher fuel consumption as compared with a normal turbocharger. In fact, this is not absolutely prohibitive because using a varioturbocharger, the engine can be made smaller for the same driving performance in the operating range mentioned. However, much more unfavorable operating conditions still exist. For example, when travelling on the level at approximately 100 km/h, p.sub.me is approximately 4 bar, n.sub.m is approximately 3,000 r.p.m. and the temperature before the turbine approximately 350.degree. C. Under these conditions, the surplus pressure .DELTA.p.sub.VT is approximately 0.6 bar, represented by the line 26-27. The boost pressure control is then at a position shortly before the opening of the varioturbocharger. Compared with a normal turbocharger, this implies excess fuel consumption of around 10%.
The disadvantage of the varioturbocharger consists in the fact that at low p.sub.me and correspondingly low temperature before the turbine, the difference between the exhaust gas pressure before the turbine and the boost pressure is greater than that with a normal turbocharger because of the higher pressure level of the varioturbocharger. In addition, the reduction of the turbine swallowing capacity is often associated with a certain deterioration in efficiency.